The inductors of the snubber network perform the second task and if these inductors are too small the current pulse during commutation can be more than the SCR’s are designed to tolerate.
The auxiliary snubber restricts the di/dt in the SCR’s of the auxiliary inverter to 28A/uSec.
When the inductance of these chokes deteriorates (often due to age), the di/dt can increase to 100A/uSec or more. The critical value for the SCR (if turned on correctly at 20deg C) is 150A/uSec. Having the di/dt at 100A/uSec under uncontrolled conditions can often be the root cause of SCR failures in auxiliary inverter assemblies.
Consider the following as an example: An SCR is conducting approximately 500 amps due to a misfire condition (unspecified reason), the SCR coming on needs to take the full 500 amps and reduce the current through the previously conducting SCR to zero before full commutation is effected. During this time both SCR’s are conducting and there is a short across 2 phases. The snubber inductors limit the rate of rise of current in the SCR coming on and isolate the effective short circuit.
The above example illustrates the importance of checking the auxiliary snubber inductors to ensure they can effectively do their job.
If an auxiliary snubber has the old style inductors (mounted on the flat), the snubber assembly is an old type and needs to be refurbished as a matter of urgency. If the auxiliary snubber has the newer (not CTI) type inductors (mounted vertically, side by side) they still need to be checked for inductance. Values of 40uH are ok and these snubber assemblies can be refurbished when convenient provided the windings are not compromised.
The 3 small flat mounted inductors start out when new at around 25 to 29uH and are ineffective when the inductance falls below 20uH. This will almost certainly be the case in trucks with 10,000 or more hours duty. The vertically mounted inductors start out when new at around 100uH. These will still almost certainly be out of specification but will still do the job.
The most significant problem with these inductors is the heat effect on the cabling which deteriorates to the point of potential break down of the primary insulation.
CTI refurbished auxiliary snubber inductors have green/yellow double cores mounted on their flat and start out with an inductance of 72 to 75uH. It is expected that these snubber inductors will last for the truck’s remaining working life, holding their inductance at above 70uH under normal conditions. They should however be checked every 1000 hour or so for insulation integrity and better than 70uH inductance. For reliable operation of the Auxiliary blower system, 40uH is a minimum.